Heat exchangers (HXs) have gained increasing attention due to the intensive demand of performance improving and energy saving for various equipment and machines. As a natural application, topology optimization has been involved in the structural design of HXs aiming at improving heat exchange performance (HXP) and meanwhile controlling pressure drop (PD). In this paper, a novel multiphysics-based topology optimization framework is developed to maximize the HXP for 2D cross-flow HXs, and concurrently limit the PD between the fluid inlet and outlet. In particular, an isogeometric analysis solver is developed to solve the coupled steady-state Navier-Stokes and heat convection-diffusion equations. Non-body-fitted control mesh is adopted instead of dynamically remeshing the design domain during the evolution of the boundary interface. The method of moving morphable voids is employed to represent and track boundary interface between the hot and the remaining regions. In addition, various constraints are incorporated to guarantee manufacturability of the optimized structures with respect to practical considerations in additive manufacturing, such as removing sharp corners, controlling channel perimeters, and minimizing overhangs. To implement the iterative optimization process, the method of moving asymptotes is employed. Numerical examples show that the HXP of the optimized structure is greatly improved compared with its corresponding initial design, and the PD between the fluid inlet and outlet is controlled concurrently. Moreover, a smooth boundary interface between the channel and the cold fluid, and improved manufacturability are simultaneously obtained for the optimized structures.

The complexity of interphase precipitation in a Fe-0.19C-1.54Mn-0.4Si-0.06Al-0.13Mo-0.06Ti (at\%) high strength low-alloy (HSLA) steel at an early stage of the austenite-to-ferrite transformation was studied by analyzing the solute distribution across ferrite-austenite interfaces with Kurdjumov-Sachs (K-S) and non-K-S orientation relationships (OR). Structural characterization i.e. ferrite/austenite OR and habit plane characteristics was performed by electron backscatter diffraction (EBSD) and the clustering back-calculation approach, while solute distributions i.e. the solute concentration spikes in the interface regions were studied by atom probe tomography (APT) on the specimens specifically prepared across/near the ferrite/austenite interfaces. It was shown for the first time that interphase precipitation is promoted at both types of interface: (i) a K-S OR and habit plane deviated from ideal (110)(alpha)//(111)(gamma) and (ii) a non K-S OR. The key aspect of interphase precipitation is the distribution of solute atoms across the interface, which is pronounced Mn, Ti, Mo and C concentration spikes at the interphase boundary. In contrast, interphase precipitates were not formed at the coherent interface with a K-S OR and habit plane of (110)(alpha)//(111)(gamma). This was correlated with the interfacial condition, where the compositional ratio of substitutional solute and solvent elements remains almost constant across the interface, i.e. Mn and C spikes. Interface compositions in this study did not match with local equilibria (negligible partitioning local equilibrium and paraequilibrium) limits. In addition, it appeared that the interfaces with Mn, Ti, Mo and C concentration spikes form ledges leading to randomly redistributed interphase precipitates. (C) 2021 Elsevier B.V. All rights reserved.

Heat exchangers (HXs) have gained increasing attention due to the intensive demand of performance improving and energy saving for various equipment and machines. As a natural application, topology optimization has been involved in the structural design of HXs aiming at improving heat exchange performance (HXP) and meanwhile controlling pressure drop (PD). In this paper, a novel multiphysics-based topology optimization framework is developed to maximize the HXP for 2D cross-flow HXs, and concurrently limit the PD between the fluid inlet and outlet. In particular, an isogeometric analysis solver is developed to solve the coupled steady-state Navier-Stokes and heat convection-diffusion equations. Non-body-fitted control mesh is adopted instead of dynamically remeshing the design domain during the evolution of the boundary interface. The method of moving morphable voids is employed to represent and track boundary interface between the hot and the remaining regions. In addition, various constraints are incorporated to guarantee manufacturability of the optimized structures with respect to practical considerations in additive manufacturing, such as removing sharp corners, controlling channel perimeters, and minimizing overhangs. To implement the iterative optimization process, the method of moving asymptotes is employed. Numerical examples show that the HXP of the optimized structure is greatly improved compared with its corresponding initial design, and the PD between the fluid inlet and outlet is controlled concurrently. Moreover, a smooth boundary interface between the channel and the cold fluid, and improved manufacturability are simultaneously obtained for the optimized structures.

Hot cracking is one of the major defects that can occur in laser-based additive manufacturing. During the terminal stage of solidification, hot cracking initiates when the semi-solid matrix builds up excessive negative (tensile) pressure induced by thermal contraction. This study presents a new quantification of the trends in the above process: we estimate the volume change brought by thermal deformation through a perspective of energy conservation and combine it with the intergranular volume change induced by grain growth and liquid backflow to derive a criterion for hot-cracking initiation. Based on this, we propose two modified indexes that build on prior work, namely: (1) vertical bar dT/d root fs vertical bar 1/root 1-beta and (2) vertical bar dT/d root fs vertical bar 1/E. Here, T is temperature, f(s) is the solid fraction of the semi-solid region, beta is the shrinkage factor and (E) over bar is the material toughness near the solidus temperature. Evaluating these indexes against experimental data reveals that hot-cracking susceptibility is strongly correlated with the second index and indeed is a function of material high-temperature toughness. (C) The Minerals, Metals \& Materials Society and ASM International 2022

Metal additive manufacturing is a disruptive technology that is revolutionizing the manufacturing industry. Despite its unrivaled capability for directly fabricating metal parts with complex geometries, the wide realization of the technology is currently limited by microstructural defects and anomalies, which could significantly degrade the structural integrity and service performance of the product. Accurate detection, characterization, and prediction of these defects and anomalies have an important and immediate impact in manufacturing fully-dense and defect-free builds. This review seeks to elucidate common defects/anomalies and their formation mechanisms in powder bed fusion additive manufacturing processes. They could arise from raw materials, processing conditions, and post-processing. While defects/anomalies in laser welding have been studied extensively, their formation and evolution remain unclear. Additionally, the existence of powder in powder bed fusion techniques may generate new types of defects, e.g., porosity transferring from powder to builds. Practical strategies to mitigate defects are also addressed through fundamental understanding of their formation. Such explorations enable the validation and calibration of models and ease the process qualification without costly trial-and-error experimentation.

Grain boundaries undergo thermally-activated, first-order transitions that result in discontinuous changes of interfacial properties. Importantly, grain boundary transitions lead to changes in bulk material prop-erties (e.g., embrittlement) and/or behavior (e.g., abnormal grain growth). Numerous studies have been completed on the equilibrium states of grain boundaries and their transitions (i.e., complexion transi-tions), but there have been far fewer investigations of complexion transition kinetics; complexion transi-tions occur on the atomic-scale and are therefore challenging to detect experimentally. In this work, a 3D Potts grain growth model with stochastic complexion transitions was employed to investigate complexion transition kinetics. A Johnson-Mehl-Avrami-Kolmogorov (i.e., JMAK) approach was used to extract nucle-ation and growth rates (i.e., transformation rates) , while point process analyses and correlation functions were used to infer complex interrelated nucleation and growth events. Time-temperature-transformation (TTT) diagrams, in particular grain-boundary complexion, transformed grain, and abnormal grain TTT di-agrams, were constructed to summarize the progress of complexion-related transformations. Such dia-grams relate complexion-induced grain growth to the underlying complexion transitions and, in the case of abnormal grain growth (AGG), permit one to assess the role of AGG as a temperature-dependent, time -displaced indicator of complexion transitions. Overall, this work details a theoretical framework that can be used to better understand complexion transition kinetics as well as to develop tools for the design of bulk microstructures.(c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Grain boundaries undergo thermally-activated, first-order transitions that result in discontinuous changes of interfacial properties. Importantly, grain boundary transitions lead to changes in bulk material prop-erties (e.g., embrittlement) and/or behavior (e.g., abnormal grain growth). Numerous studies have been completed on the equilibrium states of grain boundaries and their transitions (i.e., complexion transi-tions), but there have been far fewer investigations of complexion transition kinetics; complexion transi-tions occur on the atomic-scale and are therefore challenging to detect experimentally. In this work, a 3D Potts grain growth model with stochastic complexion transitions was employed to investigate complexion transition kinetics. A Johnson-Mehl-Avrami-Kolmogorov (i.e., JMAK) approach was used to extract nucle-ation and growth rates (i.e., transformation rates) , while point process analyses and correlation functions were used to infer complex interrelated nucleation and growth events. Time-temperature-transformation (TTT) diagrams, in particular grain-boundary complexion, transformed grain, and abnormal grain TTT di-agrams, were constructed to summarize the progress of complexion-related transformations. Such dia-grams relate complexion-induced grain growth to the underlying complexion transitions and, in the case of abnormal grain growth (AGG), permit one to assess the role of AGG as a temperature-dependent, time -displaced indicator of complexion transitions. Overall, this work details a theoretical framework that can be used to better understand complexion transition kinetics as well as to develop tools for the design of bulk microstructures.(c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

To better understand the effects of infiltration on local electrochemistry and transport in solid oxide fuel cell (SOFCs) electrodes, high-throughput, high-performance finite element simulations are presented within dozens of SOFC cathodes containing synthetically generated nanoscale infiltrates. The computational approach retains the complex microstructural morphologies of cathodes, including those of the three backbone phases (gas, ion, and electron conductors) and the infiltrates (an electron conductor), in meshed domains and computes distributions of local electrochemical quantities within the domains. Simulations were implemented on a supercomputer and converged for 48 distinct microstructural subvolumes, with varying backbone heterogeneities and infiltrate loadings. Analyzing both the ensemble (averaged over subvolumes) and the local (evaluated within subvolumes) performance metrics indicate that infiltration of an electron conductor significantly improves the electrochemical performance of each backbone in a linear fashion with the increase of triple phase boundary content, but the essential ionic transport pathways of the backbone are unchanged. These results shed into the and fabrication of electrodes in fuel cells.